This invention relates to synchronous light integrating means and methods, and more particularly to a synchronous light integrating arrangement and method for removing the adverse effect of external disturbing light (light noise), and which is, for example, highly suited for the focusing device of a camera that obtains its object distance information by projecting light onto an object to be photographed and detecting the light reflected from the object. The invention is also directed to cameras of the still, movie, and video type as well as sensors for industrial robots, and other devices using distance measurements.
Various methods and means have been proposed for these purposes. One arrangement alternately repeats a first integration mode which involves integrating light reflected from an object as a result of a light source intermittently projecting light onto the object, and a second integration mode which involves integrating the light from the object when light is not projected. The integrated value thus obtained is accumulated for each operation in the first integration mode, and the distance to the object, the reflection factor of the object, etc., are measured from the difference between the accumulated integration value obtained in the first integration mode and an accumulated integration value obtained in the second integration mode. Such a synchronous integrating arrangement appears in the camera distance detecting device disclosed, for example, in Japanese Laid-Open Patent Application No. Sho 54-151829. Here, light is projected on an object to be photographed, the resultant light reflected from the object is detected to obtain distance information, and a photo-taking lens is adjusted to focus on the object. An active focusing device of this type, which projects light, requires separating the component of the reflected light arising from the projected light and the component resulting from ambient light sources such as the sun, artificial lighting, etc., to detect the projected light. This process may be called external light removal, or light noise removal.
The light projected onto the object is dispersed over the object. The quantity S of projected light, as detected by the detector of the focusing device, is inversely proportional to the square of the object distance R. Accordingly, in order to focus on a far away object, the projected light must be accurately detected. The detection accuracy hinges upon the removal of the external light or light noise. The external light removal is thus an important factor in focusing accuracy.
Generally, the external light has a large DC component and substantial fluctuations as compared with the projected light. The fluctuations result from the flicker of the external light source and movement of objects including those objects to be photographed. The salient cause of light source flicker is that inherent in the fluctuating voltage of an electric or fluorescent lamp. It includes many frequency components of 100 Hz or 120 Hz.
The aforementioned prior art attempts to remove the external light component for accurate detection of the projected signal light component by employing a synchronous integration method. Here, the projector blinks at a relatively high frequency to produce an intermittent signal light. Suitable means accumulate detected signal V1, which is obtained between projections of signal light, and detection signal V2 which is obtained while the signal light is being projected. A difference D between the signal V1 and V2 represents the quantity or value of the projected signal light. Assuming that a detected signal obtained at the j-th cycle of accumulation between signal light projections is V1j, and accumulated signal between light projections is V1, a detected signal obtained for the j-th cycle accumulation during signal light projection is V2j, and the accumulated signal obtained while signal light is projected is V2, the signals may be expressed as follows: ##EQU1##
The accumulated signal V1 represents only the external light, and the other accumulated signal V2, the combination of the external light and the signal light. Therefore, if the quantity of external light is assumed to be VE, and that of the signal light to be s, the detected signal V1j obtained between signal light projections and the detected signal V2j obtained during signal light projection can be expressed as shown: EQU V1j=VEj (with the signal light not projected) EQU V2j=VEj'+s (with the signal light projected)
What is to be obtained is the sum S of the signal component sj (=s). The sum signal S can be expressed as follows: ##EQU2##
Meanwhile, the difference signal D can be expressed as follows: ##EQU3##
The difference E between the difference signal D and the sum signal S thus represents an error in detecting the signal light, and can be expressed as follows: ##EQU4##
If the external light VE varies relatively slowly and the VEj'-VEj is small, the signal light detection error becomes small. This is as shown in FIG. 4. Here hatched parts VE1 and VE2 . . . represent the values obtained with no signal light projected, i.e., between projected signal light and cross-hatched parts VE1' and VE2' . . . represent the values obtained with projected signal light. Other drawings are arranged in the same manner. However, where the quantity y of the external light varies linearly as shown in FIG. 5, the detection error E obtained after N accumulations is: ##EQU5##
The error E is proportional to the number of times N. Assuming that the signal light projection period is .tau.c and the duty ratio of the detection signal integration time at each accumulating cycle or step is X, the integration time per accumulating step becomes X.tau.c. Assuming that the value of the duty ratio X is unvarying, the error h of the external light per signal light projection period becomes smaller in proportion to the period .tau.c when the signal light projection frequency is arranged to be faster or higher. Then, according to this, the quantity sj of the signal light becomes smaller in proportion to the signal light projection period .tau.c. This can be expressed as follows: ##EQU6##
Then, the relationship between S and E S/E can be expressed as follows: EQU S/E=s/h=k2/k1=const.
Thus, the ratio S/E is irrelevant to the value .tau.c and N. Therefore, where the external light varies linearly, conventional synchronous integrating systems are incapable of removing the linear variation. Further, the external light removal is not adequately accomplished with fluctuations at the relatively low frequency 100 Hz or 120 Hz of a fluorescent lamp, etc.
An object of the invention is to improve light integrating circuits, methods, distance measuring devices, cameras and other devices.
Another object of the invention is to eliminate the shortcomings of the prior art.
A general object of the invention is provide a synchronous integrating method and apparatus which is capable of accurately removing the external light component.